Anti-listeria bacteriocin

ABSTRACT

The invention concerns an isolated polypeptide which is a bacteriocin called Sakacin G derived from  Lactobacillus sakei  2512. The invention also concerns a nucleic acid molecule encoding for said bacteriocin and the use of said polypeptide as an active agent against pathogenic and undesirable flora in the preparation of food products.

This application is a national stage application of PCT/FR01/01642, filed May 28, 2001, which claims priority to French applications 00/06859, filed May 29, 2000 and 00/13407, filed Oct. 19, 2000.

The present invention relates to a bacteriocin of Lactobacillus sakei and more especially of Lactobacillus sakei 2512, to a nucleotide sequence coding for that bacteriocin, and to the industrial use of that bacteriocin as an active agent against pathogenic or undesirable flora in the preparation of food products.

Lactic acid bacteria are used intensively in the fermentation of foods not only to improve the taste and texture of the foods but especially to prolong their storage life. Numerous lactic acid bacteria are in fact capable of inhibiting the growth of certain Gram positive bacteria, including pathogenic strains such as Listeria monocytogenes, thanks to the excretion of antagonistic molecules, including peptide compounds. These peptide compounds, called bacteriocins, are therefore potentially valuable for preserving fermented food products in terms of quality and health.

As examples of such bacteriocins, special mention may be made of those which form the sub-class of polypeptides called anti-Listeria bacteriocins, bacteriocins of class IIa (Ennahar S. et al., 2000, FEMS Microbiol. Rev., 24: 85-106) and cystibiotic bacteriocins (Jack R. et al., 1995, Microbiol. Rev., 59(2): 171-200). The potential use of one of these class IIa bacteriocins, divercin V41, for preventing the growth of Listeria monocytogenes in smoked salmon has recently been noted (Duffes F. et al., 1999, J. Food Prot., 62(12): 1394-1403).

The sequences of these polypeptides exhibit strong similarities in the N-terminal portions, with the presence of a disulfide bridge in particular. The hydrophobic C-terminal portion is much more variable, but some of those bacteriocins, so-called pediocin-type bacteriocins (pediocin PA-1, enterocin A and divercin V41), are characterised by a number of residues greater than 40 and the presence of a second disulfide bridge on the C-terminal side.

The authors of the present invention have discovered a new class IIa bacteriocin produced from a specific strain of Lactobacillus sakei, which proves to be especially effective in inhibiting the growth of Listeria, more especially of Listeria monocytogenes.

In agreement with Tagg J. R. et al., Bacteriol. Rev., 40: 722-756 (1976), the term “bacteriocin” within the scope of the invention refers to a polypeptide produced, by ribosome synthesis, from microorganisms capable of inhibiting specifically the growth of other bacteria.

The present invention therefore relates in the first instance to a polypeptide derived from the strain Lactobacillus sakei 2512, having bacteriocin activity.

The strain Lactobacillus sakei 2512 was deposited on 25th May 2000 with the Collection Nationale des Cultures de Microorganismes (National Collection of Microorganism Cultures), where it is registered under deposit number I-2479.

The bacteriocin to which the present invention relates has been named sakacin G. It is a polypeptide having a molecular mass of the order of from 3700 to 3900 and preferably of about 3834 Da, determined by mass spectrometry. It has a bacterial inhibition spectrum which is very similar to that of the class IIa bacteriocins. Accordingly, it proves to be especially effective against the strains of Lactobacillus sakei other than Lactobacillus sakei 2512, Pediococcus cerevisiae, the totality of the Listeria strains and against Enterococcus faecalis and Enterococcus durans. By contrast, it proves to be inactive against the other species of Lactobacillus such as, for example, Lactobacillus debrueckii, Lactobacillus plantarum, Lactobacillus brevis, Lactobacillus casei and a strain of Enterococcus faecium.

Like the anti-Listeria bacteriocins of the pediocin type, sakacin G advantageously possesses two disulfide bridges in its peptide structure.

An analysis of the genetic determinants of several class IIa bacteriocins has shown that the genes involved in their production, transport and immunity are organised into one or more operon-type structures. These operons are often located in a plasmid and generally possess at least two genes coding for proteins, homologous to an ABC transporter and an accessory protein, probably involved in bacteriocin export.

Cloning of the nucleotide fragment containing the sakacin G gene has revealed the existence of three complete open reading frames skgA1 (SEQ ID NO: 1), skgA2 (SEQ ID NO: 3) and skgDc (SEQ ID NO: 13) (including the truncated reading frame skgD (SEQ ID NO: 7)) and a truncated frame skgI (SEQ ID NO: 5), a diagrammatic representation of which is shown in FIG. 1. The nucleotide fragment is a double strand, the 5′-3′ single strand of which is shown in SEQ ID NO: 15.

The products of the genes skgA1 and skgA2, called pre-bacteriocins, may undergo maturation during which their respective leader peptides are cleaved between residues 18 and 19, thus liberating active sakacin G (residues 19-55).

The 5′-3′ single-strand nucleotide fragment comprising skgA1, skgA2, skgD and skgI appears in SEQ ID NO: 9.

The present invention accordingly relates also to an isolated polypeptide corresponding to a bacteriocin, characterised in that it comprises SEQ ID NO: 2 and/or SEQ ID NO: 4. The sequence of the mature bacteriocin corresponds to SEQ ID NO: 12 and is comprised in sequences ID NO: 2 and ID NO: 4.

The reading frame called skgI codes for a protein of 52 residues. A comparison of that sequence with the database sequence shows strong similarities between SkgI and so-called immunity proteins. It probably codes for the immunity protein protecting the sakacin-G-producing bacterium.

The present invention extends also to an isolated polypeptide comprising SEQ ID NO: 6 corresponding to the reading frame skgI.

With regard to the last gene skgDc, it codes for a protein which is homologous with proteins of the ABC transporter family, and more especially of the transporter of pediocin PA-1. The gene skgDc probably codes for the ABC transporter specific to sakacin G.

The present invention extends also to the isolated polypeptide comprising SEQ ID NO: 8 corresponding to the so-called skgD gene, and to the isolated polypeptide comprising SEQ ID NO: 14 corresponding to the so-called skgDc gene.

It will be understood that homologous sequences are also included, which sequences are defined as

-   i) sequences that are similar to at least 70% of sequence SEQ ID NO:     2, NO: 4, NO: 6, NO: 8, NO: 12 or NO: 14; or -   ii) sequences coded for by a homologous nucleic acid sequence as     defined hereinbelow, that is to say a nucleic acid sequence that     hybridises with sequence SEQ ID NO: 1, NO: 3, NO: 5, NO: 7, NO: 9,     NO: 13 or NO: 15 or its complementary sequence, under stringent     hybridisation conditions.

There too, the term “similar” refers to perfect resemblance or identity between the amino acids of the homologous sequences under comparison, but also to non-perfect resemblance, which is referred to as similarity. This search for similarities in a polypeptide sequence takes into account conservative substitutions, which are substitutions of amino acids of the same class, such as substitutions of amino acids in non-charged side chains (such as asparagine, glutamine, serine, threonine and tyrosine), of amino acids in basic side chains (such as lysine, arginine, histidine), of amino acids in acid side chains (such as aspartic acid and glutamic acid); of amino acids in non-polar side chains (such as glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan and cysteine).

More generally, therefore, “homologous amino acid sequence” is understood as being any amino acid sequence that differs from sequence SEQ ID NO: 2, NO: 4, NO: 6, NO: 8, NO: 12 or NO: 14 by substitution, deletion and/or insertion of an amino acid or of a reduced number of amino acids, especially by substitution of natural amino acids by non-natural amino acids or pseudo-amino acids in positions such that these modifications do not significantly affect the biological activity of the isolated polypeptide and preferably of sakacin G.

Such a homologous amino acid sequence is preferably similar to at least 85% of sequence SEQ ID NO: 2, NO: 4, NO: 6, NO: 8, NO: 12 or NO: 14, preferably at least 95%.

Homology is generally determined using sequence analysis software (for example Sequence Analysis Software Package of the Genetics Computer Group, University of Wisconsin Biotechnology Center, 1710 University Avenue, Madison, Wis. 53705). Similar amino acid sequences are aligned in order to obtain the maximum degree of homology (i.e. identity or similarity, as defined above). To that end, it may be necessary to introduce gaps into the sequence artificially. Once optimum alignment has been achieved, the degree of homology is established by recording all the positions in which the amino acids of the two sequences under comparison are identical, relative to the total number of positions.

The biological activity of the isolated polypeptide, and especially of sakacin G, refers to its capacity to inhibit the growth of undesirable and/or pathogenic bacterial strains, preferably of Listeria bacteria and more especially of Listeria monocytogenes bacteria.

The present invention relates also to an isolated nucleic acid coding for a polypeptide as defined above.

More precisely, the present invention relates to an isolated nucleic acid comprising SEQ ID NO: 1 and/or SEQ ID NO: 3.

The complete nucleotide sequence of the region involved in the expression of sakacin G (3055 bp) has been determined. It is a double-strand DNA whose 5′-3′ strand is shown in SEQ ID NO: 15. The 3′-5′ strand is shown in FIG. 2. The present invention relates also to a nucleic acid comprising such a sequence.

As described above, this sequence has three complete open reading frames skgA1, skgA2 and skgDc and a truncated frame skgI. The supposed genes skgA1 (SEQ ID NO: 1), skgA2 (SEQ ID NO: 3) and skgI (SEQ ID NO: 5) are oriented therein in the opposite direction relative to skgDc (SEQ ID NO: 13).

Within the scope of the present invention there are also claimed the nucleic acid comprising SEQ ID NO: 5, the nucleic acid comprising SEQ ID NO: 13 and the nucleic acid comprising SEQ ID NO: 7.

It will be understood that homologous sequences are also included, which sequences are defined as:

-   i) sequences that are similar to at least 70% of sequence SEQ ID     NO:1, NO: 3, NO: 5, NO: 7, NO: 9, NO:13 or NO: 15; or -   ii) sequences that hybridise with sequence SEQ ID NO: 1, NO: 3, NO:     5, NO: 7, NO: 9, NO: 13 or NO: 15 or their complementary sequence,     under stringent hybridisation conditions; or -   iii) sequences coding for the polypeptide named sakacin G, as     defined above.

A homologous nucleotide sequence according to the invention is preferably similar to at least 75% of the sequence SEQ ID NO: 1, NO: 3, NO: 5, NO: 7, NO: 19, NO: 13 or NO: 15, preferably at least 85% or at least 90%.

Such a homologous nucleotide sequence preferably hybridises specifically with the complementary sequences of sequence SEQ ID NO: 1, NO: 3, NO: 5, NO: 7, NO: 9, NO: 13 or NO: 15 under stringent conditions. The parameters defining the stringency conditions depend on the temperature at which 50% of the coupled strands separate (Tm).

For sequences comprising more than 30 bases, Tm is defined by the equation (Sambrook et al., 1989, NY: Cold Spring Harbor Laboratory): Tm=81.5+0.41(% G+C)+16.6 Log(cation concentration)−0.63(% formamide)−(600/number of bases)

For sequences having a length less than 30 bases, Tm is defined by the equation: Tm=4(G+C)+2(A+T).

Under appropriate stringency conditions, under which the aspecific sequences do not hybridise, the hybridisation temperature may preferably be from 5 to 10° C. below Tm, and the hybridisation buffers used are preferably solutions of high ionic strength, such as a 6×SSC solution, for example.

The expression “similar sequences” used above refers to perfect resemblance or identity between the nucleotides under comparison, but also to non-perfect resemblance, which is referred to as similarity. This search for similarities in nucleic sequences distinguishes, for example, purines and pyrimidines.

A homologous nucleotide sequence having the open reading frames shown in SEQ ID NO: 1, NO: 3, NO: 5, NO: 7, NO: 9, NO: 13 or NO: 15 therefore includes any nucleotide sequence which differs from sequence SEQ ID NO: 1, NO: 3, NO: 5, NO: 7, NO: 9, NO: 13 or NO: 15 by mutation, insertion, deletion or substitution of one or more bases, or by the degeneracy of the genetic code, insofar as it codes for a polypeptide having the biological activity of sakacin G, as defined hereinbelow.

Such homologous sequences include sequences of the genes of bacteria other than Lactobacillus, coding for sakacin G.

The polypeptides of the present invention can be synthesised by any method known to the person skilled in the art. The polypeptides of the invention may, for example, be synthesised by techniques of the chemistry of synthesis, such as Merrifield-type synthesis, which is advantageous for reasons of purity, of antigen specificity, of the absence of undesirable secondary products and of ease of production.

The present invention relates also to a process for the production of a recombinant polypeptide, in which a vector comprising a nucleic acid according to the present invention is transferred into a host cell which is cultured under conditions permitting the expression of a polypeptide according to the present invention or of a polypeptide coded for by a nucleic acid sequence according to the present invention.

The recombinant bacteriocin may also be produced by a process in which a vector containing a nucleic acid comprising a nucleotide sequence according to the invention, and preferably the sequences SEQ ID NO: 1 and/or NO: 3 or a homologous sequence, is transferred into a host cell which is cultured under conditions permitting the expression of the corresponding polypeptide. The resulting protein can then be recovered and purified. The purification processes used are known to the person skilled in the art. The resulting recombinant polypeptide can be purified starting from lysates and cell extracts, from the supernatant of the culture medium, by methods used individually or in combination, such as fractionation, methods of chromatography, techniques of immunoaffinity with the aid of specific monoclonal or polyclonal antibodies, etc.

The nucleic acid sequence of interest, coding for sakacin G, can be inserted into an expression vector in which it is linked in an operative manner to elements permitting regulation of its expression, such as, especially, promoters, activators and/or transcription terminators. The signals controlling the expression of the nucleotide sequences (promoters, activators, termination sequences, etc.) are chosen depending on the cell host used. To that end, the nucleotide sequences according to the invention can be inserted into vectors which replicate autonomously within the chosen host, or vectors which integrate in the chosen host. Such vectors will be prepared by the methods conventionally used by the person skilled in the art, and the clones resulting therefrom can be introduced into a suitable host by standard methods, such as, for example, electroporation or calcium phosphate precipitation.

The cloning and/or expression vectors as described above, containing a nucleotide sequence defined according to the invention, also form part of the present invention.

The invention relates also to the host cells transformed, temporarily or permanently, by those expression vectors. These cells can be obtained by introducing into host cells, preferably prokaryotic host cells, a nucleotide sequence inserted into a vector as defined above, then culturing said cells under conditions permitting the replication and/or expression of the transferred nucleotide sequence.

Examples of host cells include especially bacteria such as Lactococcus, Lactobacillus, Leuconostoc, Streptococcus, Pediococcus, Escherichia and yeasts.

The nucleotide sequences of the invention may be of synthetic or natural origin. They may be DNA or RNA sequences obtained by the screening of sequence libraries by means of probes produced on the basis of the sequences SEQ ID NO: 1, NO: 3, NO: 5, NO: 7, NO: 9, NO: 13 and/or NO: 15. Such libraries can be prepared by conventional techniques of molecular biology known to the person skilled in the art.

The nucleotide sequences according to the invention can also be prepared by chemical synthesis, or alternatively by mixed methods including the chemical or enzymatic modification of sequences obtained by the screening of libraries.

The present invention relates also to a method of inhibiting the growth of Listeria, more especially of Listeria monocytogenes, in an environment which may or may not be a food environment and which is susceptible to contamination by Listeria monocytogenes.

Listeria monocytogenes are pathogenic microorganisms which are the source of severe diseases in humans and animals and which especially can easily be transmitted by contaminated foods, more especially by means of meat, meat products, seafood products, milk and products derived therefrom. The present invention therefore proposes a method of inhibiting the growth of Listeria monocytogenes in a food likely to contain Listeria monocytogenes as a contaminant, said process comprising the addition of a polypeptide according to the invention to said food in an amount sufficient to inhibit the growth of Listeria monocytogenes.

The bacteriocins according to the invention are preferably used in any food system in an amount of from 1 to 100,000 arbitrary units (AU) of bacteriocins per gram of food.

An AU of bacteriocins is defined as 5 μl of the highest dilution of the culture supernatant leading to a defined growth inhibition zone, relative to a control strain of a Gram positive bacteria, on an agar medium.

Although foods are most affected by Listeria monocytogenes contamination, veterinary and medical products can also be contaminated with this type of bacteria, as can cosmetic products or similar products.

The bacteriocins according to the present invention, and especially sakacin G, can therefore also be used to inhibit the growth of this type of pathogen in such products.

Accordingly, the present invention relates to the use of a bacteriocin according to the present invention as an active agent against pathogenic or undesirable flora, especially in the preparation of food products, and more precisely to inhibit the growth and propagation of Listeria, more especially of Listeria monocytogenes, in food products.

The polypeptide can be incorporated as such into the food product in question, or alternatively it can be produced therein from the strain Lactobacillus sakei 2512.

Accordingly, the present invention relates also to the use of the strain Lactobacillus sakei 2512 in a food product to generate therein a bacteriocin polypeptide according to the invention.

The invention relates also to a bacteriocin composition, characterised in that it comprises at least one polypeptide according to the present invention, that is to say derived from the strain Lactobacillus sakei 2512 or comprising sequence SEQ ID NO: 2 or NO: 4 or NO: 12 or NO: 14 or the strain Lactobacillus sakei 2512.

The invention extends also to the use of the strain Lactobacillus sakei 2512 intended to produce a polypeptide as defined above, for inhibiting the growth and propagation of Listeria, more especially of Listeria monocytogenes, in food products, and the compositions comprising that strain.

The Examples and the Figure below are given by way of example and do not limit the subject-matter of the present invention.

FIGURE

FIG. 1: Diagrammatic representation of the genetic locus involved in the production of sakacin G.

FIG. 2: Complementary 3′-5′ strand corresponding to the complete nucleotide sequence of the region involved in the expression of sakacin G and the 5′-3′ strand of which is shown in SEQ ID NO: 15.

MATERIALS AND METHODS

-   Bacterial strains and culture media. Lactobacillus sakei 2512 is     cultured at 30° C. in MRS medium (DIFCO Laboratories) sterilised for     12 minutes at 110° C. The indicator strains are cultured in BHI     medium (brain-heart infusion; DIFCO Laboratories) at 37° C. -   Test of activity. BHI medium, supplemented with 10 g/l agar, is     inoculated at 1% with a preculture of the indicator strain in     stationary phase before being poured into a Petri dish. 50     microliters of sakacin G solution are placed in wells formed in     cooled agar with a hole punch. The bacteriocin activity manifests     itself in the appearance of inhibition zones around the wells after     incubation overnight at 37° C. -   Protein analysis. Sakacin G is analysed by mass spectrometry using a     Perkin-Elmer Sciex API 165 device equipped with an Ionspray     ionisation source. After lyophilisation, the active HPLC fraction is     taken up in an acetonitrile/water solution (1:1) containing 0.1%     formic acid, and then injected by infusion at a rate of 5 μl/minute.

The protein concentration is determined by the bicinchoninic acid method by means of the BCA kit (Sigma) according to the manufacturer's instructions.

Protein sequence comparisons are carried out using the BLAST (1) program, obtainable from the ExPASy server of the Swiss Institute of Bioinformatics.

-   Molecular cloning and transformation. The plasmids are extracted and     purified from strains of Escherichia coli and Lactobacillus sakei     2512 according to the methods previously described by Sambrook et     al., 1989, NY: Cold Spring Harbor Laboratory and Muriana and     Klaenhammer, 1987, Appl. Environ. Microbiol., 53: 553-560,     respectively.

The DNA restriction and modification enzymes are used according to the supplier's instructions (Gibco-BRL). Analytical and preparative agarose gel electrophoreses are carried out in Tris/borate/EDTA buffer (pH 8.3) according to the methods described by Sambrook et al., 1989, NY: Cold Spring Harbor Laboratory. The digested DNA fragments are purified starting from agarose gels using the “Prep-a-Gene” kit (Bio-Rad). Cloning in plasmids pGEM-T (Promega) and pZERO2 (Invitrogen) is carried out in accordance with the suppliers' recommendations. Southern-type transfer is carried out on nylon membrane (Hybond-N+, Amersham) according to Sambrook et al., 1989, NY: Cold Spring Harbor Laboratory. The transfer is followed by hybridisation using a radioactive probe obtained by ³²P labelling with the aid of a “random primers DNA labelling system” kit (Gibco-BRL). The E. coli bacteria are rendered competent and transformed according to the method of Hanahan, 1983, J. Mol. Biol. 166: 557-580.

Taq polymerase (Gibco-BRL) is used according to the supplier's recommendations. Amplification of the DNA fragment coding for sakacin G was carried out with the aid of a “Geneamp 9700®” device (Perkin-Elmer) under the following conditions: 35 denaturing cycles at 94° C. for 30 seconds, hybridisation at 45° C. for 30 seconds and elongation at 72° C. for 1 minute, followed by an additional elongation cycle at 72° C. for 5 minutes.

The DNA fragment carrying the sakacin G locus is sequenced with the aid of an ABI Prism 310® automatic sequencer (Perkin-Elmer) using the “Big-dye terminator®” sequencing kit (Perkin-Elmer) and the appropriate nucleotide primers.

EXAMPLE 1 Isolation and Purification of Sakacin G

A 16 h culture of Lactobacillus sakei 2512 (100 ml) is centrifuged at 6000 g for 15 minutes. The culture supernatant is then heated at 70° C. for 20 minutes. The cooled supernatant is then diluted with 1 volume of water (the pH of the diluted solution must be below 6, by addition of 1M HCl if necessary) before being passed over a column (2.5×18 cm) containing a cation-exchange resin (carboxymethylcellulose; Cellufine C-200, Amicon) equilibrated with water. After washing in succession with water (100 ml) and then with a 0.1M NaCl solution (150 ml), the sakacin G is eluted with a 0.5M NaCl solution (200 ml). The pH of all the solutions must be below 6. The active fraction is then deposited on a solid-phase extraction cartridge (Sep-pak plus C18, Waters) equilibrated in water. After washing in succession with 5 ml of 20 mM ammonium acetate solutions containing 0, 10, 20 and 30% acetonitrile, the sakacin G is eluted with 10 ml of 20 mM ammonium acetate containing 80% acetonitrile. After lyophilisation, the extract is dissolved in 1 ml of 40% aqueous acetonitrile solution and then injected onto a C8 reverse phase analytical HPLC column (Kromasil, 5 μm, 100 Å, 4.6×250 mm, A.I.T.). The HPLC was carried out on an apparatus comprising a Perkin-Elmer series 200 LC pump connected to a Perkin-Elmer 785A detector. The absorption chromatogram is recorded at 220 nm. Separation is carried out, at a rate of 0.8 ml/minute, according to the following gradient: solvent A=water/0.1% trifluoroacetic acid; solvent B=acetonitrile/water/0.07% trifluoroacetic acid. After washing for 5 minutes with 20% of solvent B, elution is carried out by a gradient of from 20 to 40% of solvent B in 10 minutes then from 40 to 55% of solvent B in 20 minutes.

The fraction corresponding to the peak at 23 minutes proved to be active against Listeria ivanovii BUG 496 and was analysed by “ionspray” ionisation mass spectrometry. The molecule appears at least 95% pure and has a molecular mass of 3834.32±0.31 Da. The quantity of sakacin G so purified was estimated at 120 μg from 100 ml of culture. The purification yield was estimated at 55% of activity found. Part of the primary sequence of sakacin G was determined by microsequencing and two degenerate oligonucleotides were established starting from that sequence.

EXAMPLE 2 Cloning of the Genetic Locus Involved in the Production of Sakacin G

By reverse genetics, two degenerate oligonucleotides SakG01 (5′ AARTATTATGGNAAYGGNGT 3′) (SEQ ID NO: 10) and SakG02S (5′ ACATGATGNCCNCCRTTNGC 3′) (SEQ ID NO: 11) were chosen in order to amplify the DNA fragment corresponding to the structural gene of mature sakacin G (SEQ ID NO: 15) by polymerase chain reaction (PCR). The amplified product so obtained, having an approximate size of 100 bp, was cloned in plasmid pGEM-T to form plasmid pJMBYC01. The restriction fragment PvuII of 560 bp, derived from pJMBYC01, including the inserted fragment, was used as the hybridisation probe during a Southern-type transfer for locating the structural gene on the genome of Lactobacillus sakei 2512. Starting from a plasmid extract of Lb. sakei 2512 digested by the restriction enzymes HindIII and EcoRI, the probe revealed fragments having sizes of approximately 2.1 and 9 kbp, respectively. The fragment HindIII of 2.1 kbp was purified and then inserted into the vector pZERO2 in order to yield the plasmid pJMBYC02. The presence of the structural gene of sakacin G in pJMBYC02 was demonstrated by PCR amplification with the primers SakG01 and SakG02 and then by nucleotide sequencing of the fragment inserted in pJMBYC02. A similar strategy was used to determine the complete sequence of the gene skgD. The plasmid extract of Lb. sakei 2512 was digested by XbaI. The digestion product was inserted into plasmid pBluescript SK+. The clones carrying the sequence of interest were revealed by means of a radioactive probe prepared by PCR carried out on plasmid pJMBYC02 with the aid of the oligonucleotides SakG03 (5′ CCTTGGTCAGGCTATCG 3′) (SEQ ID NO: 16) and SakG04 (5′ ATCACCTTTTTGAATTACCC 3′) (SEQ ID NO: 17).

Analysis of the complete nucleotide sequence of the region (3051 bp) revealed the existence of three complete open reading frames skgA1 and skgA2 and skgDc and a truncated frame skgI. The supposed genes skgA1, skgA2 and skgI are oriented in the opposite direction relative to skgD.

Each of the open reading frames is preceded by a potential ribosome fixing site. The genes skgA1 and skgA2 both code for proteins having 55 amino acid residues, the sequences 19-55 of which are completely identical. Sequence 19-52 corresponds to the sequence of sakacin G obtained by microsequencing. The presence of 4 cysteine residues in positions 9, 14 and 24 and in the C-terminal position is to be noted. Moreover, the calculated molecular mass of this peptide, 3838.2 Da, which differs from the measured molecular mass (3834.32 Da) by 4 Da, shows the presence of two disulfide bridges in sakacin G, as has already been demonstrated for other anti-Listeria bacteriocins.

Sequences 1-18 of the proteins SkgA1 and SkgA2 differ by only 3 residues and have strong homologies with the “leader” peptides of the class II bacteriocins, which are involved in the transport of those peptides by specific ABC transporters. In particular, the terminal GG unit is characteristic of these leader sequences and constitutes the maturation site of these bacteriocins. A comparison of the nucleotide sequences of the genes skgA1 and skgA2 also shows an identity of sequence of more than 95% for the portion of the genes coding for the mature bacteriocin.

The incomplete open reading frame called skgI codes for a protein of 52 residues. A comparison of that sequence with the database sequences shows strong homologies between SkgI and the so-called immunity proteins LccI and MesI. The involvement of MesI in protection with respect to mesentericin Y105 has been demonstrated. It may be assumed that skgI codes for the sakacin G immunity protein.

The last gene skgDc codes for a protein of 727 amino acids. According to the databases, SkgDc is highly homologous with proteins of the ABC transporter family and more especially with transporters of pediocin PA-1: PedD or PapD (Marugg et al., 1992; Appl. Environ. Microbiol. 58, 2360-2367; Motlagh et al., 1994, Lett. Appl. Microbiol. 18, 305-312), of sakacin P: SppT (Huhne et al., 1996, Microbiology 142, 1437-1448), of sakacin A: SapT (Axelsson and Holck, 1995, J. Bacteriol. 177, 2125-2137) and of mesentericin Y105: MesD (Fremaux et al., 1995, Microbiology 141, 1637-1645).

EXAMPLE 3 Inhibition Spectrum

The sakacin G sensitivity of 17 bacterial strains was tested by the well test method (see Materials and Methods). The results are shown in Table 1 below:

TABLE 1 Radius of the inhibition halos (mm) Lc. lactis ATCC11454 0 Ln. Paramesenteroides DSM 20288 0 Ln. Mesenteroides DSM 20484 0 Ln. Mesenteroides DSM 20240 0 Lb. Delbrueckii DSM 20081 0 Lb. Plantarum DSM 20174 0 Lb brevis DSM 20054 0 Lb. casei DSM 20011 0 Lb. sakei 2515 1 P. acidilactici ENSAIA 583 0 P. cerevisiae IP 5492 1 E. faecium ENSAIA 631 0 E. faecalis IP 5430 2 E. faecalis ENSAIA 636 1 E. durans ENSAIA 630 2 L. inocua 8811 3 L. ivanovi BUG 496 6

The inhibition spectrum of this bacteriocin appears to be quite narrow and limited to the strains Lactobacillus sakei and Pediococcus cerevisiae for the lactic acid bacteria. Like the other class IIa bacteriocins, this peptide appears to be active against all the Listeria strains tested, as well as against Enterococcus faecalis and Enterococcus durans, but not against Enterococcus faecium. 

1. An isolated sakacin G polypeptide having bacteriocin activity, wherein said polypeptide is derived from Lactobacillus sakei
 2512. 2. The isolated polypeptide according to claim 1, wherein said polypeptide it is a class IIa bacteriocin.
 3. An isolated sakacin G polypeptide having bacteriocin activity, wherein said polypeptide comprises SEQ ID NO: 2 and/or SEQ ID NO:
 4. 4. An isolated sakacin G polypeptide having bacteriocin activity, wherein said polypeptide comprises SEQ ID NO:
 12. 5. A composition comprising the polypeptide of any one of claims 1 to
 4. 6. An isolated nucleic acid molecule comprising a nucleotide sequence encoding the polypeptide according to any one of claims 1 to
 4. 7. The nucleic acid molecule according to claim 6, said nucleic acid molecule comprising SEQ ID NO: 1 and/or SEQ ID NO:
 3. 8. The nucleic acid molecule according to claim 6 or 7, said nucleic acid molecule comprising SEQ ID NO:
 15. 9. The nucleic acid molecule according to claim 6 or 7, said nucleic acid molecule comprising SEQ ID NO:
 9. 10. An isolated sakacin G polypeptide having bacteriocin activity, wherein said polypeptide comprises SEQ ID NO:
 6. 11. An isolated sakacin G polypeptide having bacteriocin activity, wherein said polypeptide comprises SEQ ID NO:
 8. 12. An isolated sakacin G polypeptide having bacteriocin activity, wherein said polypeptide comprises SEQ ID NO:
 14. 13. An isolated nucleic acid molecule comprising SEQ ID NO: 5 and encoding a polypeptide having bacteriocin activity.
 14. An isolated nucleic acid molecule comprising SEQ ID NO: 7 and encoding a polypeptide having bacteriocin activity.
 15. An isolated nucleic acid molecule comprising SEQ ID NO: 13 and encoding a polypeptide having bacteriocin activity.
 16. A cloning and/or expression vector comprising the nucleic acid molecule according to any one of claims 6 to 7, 13, or
 15. 17. A host cell transformed by the vector according to claim
 16. 18. The host cell according to claim 17, wherein said host cell is a microorganism selected from the group consisting of Lactococcus, Lactobacillus, Leuconostoc, Streptococcus, Pediococcus, Escherichia, and yeast.
 19. A method for producing sakacin G polypeptide, wherein said method comprises culturing the host cell of claim 17 or 18 under conditions to express said protein, and isolating said protein.
 20. A method for inhibiting the growth and propagation of Listeria in food products, said method comprising administer to said food product the polypeptide of any one of claims 1 to
 4. 21. The method of claim 20, wherein said Listeria is Listeria monocytogenes.
 22. The method of claim 20, wherein said polypeptide is produced in the food product from the stain Lactobacillus sakei
 2512. 